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Ethernet

Ethernet. Ethernet and IEEE 802.3. Developed during the mid-1970 抯 at Xerox Palo Alto Research Center (Bob Metcalfe) Later refinement by DEC, Intel and Xerox (DIX standard) Became IEEE 802.3 in 1985 Different frame format between Ethernet and IEEE 802.3. Ethenet architecture.

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Ethernet

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  1. Ethernet

  2. Ethernet and IEEE 802.3 • Developed during the mid-1970抯 at Xerox Palo Alto Research • Center (Bob Metcalfe) • Later refinement by DEC, Intel and Xerox (DIX standard) • Became IEEE 802.3 in 1985 • Different frame format between Ethernet and IEEE 802.3

  3. Ethenet architecture Data link layer Physical layer station interface data encapsulation NIC link management encoding and decoding AUI cable transmission and receipt transceiver tap BNC connector 0.5 ?Coax

  4. Multi segments network 10Base2 - Thin Ethernet hub bridge 10Base5 - Thick Ethernet hub router server 10BaseT-Twisted pair

  5. IEEE 802.3 specification • Various standard defined for IEEE802.3 with a concise notation • 10Base5 -- thickwire coaxial • 10Base2 -- thinwire coaxial or cheapernet • 10BaseT -- twisted pair • 10BaseF -- fiber optics • 10Broad36 -- broadband • Fast Ethernet • 100BaseTX, 100BaseT4, 100BaseF and 100 VG-AnyLAN 10 Base 5 maximum segment length in hundreds of meters data rate in Mbps signaling baseband or broadband

  6. vampire tap 0.5?Coax BNC connector transceiver AUI cable NIC 10Base5 • tap : cable does not to be cut • transceiver : send/receive, collison • detection, electronics isolation • AUI : Attachment Unit Interface • Use for backbone networks maximum segment length=500m maximum number of stations per segment=100 minimum distance between two stations = 2.5 m maximum network distance between two stations = 2.8km

  7. 10Base2 0.25 ?Coax • BNC connector • No drop cable • use for office LAN BNC T-connector NIC maximum segment length=185m maximum number of stations per segment=30 minimum distance between two stations = 0.5 m maximum network distance between two stations = 925 m

  8. 10BaseT • A hub functions as a repeater • use for office LAN hub NIC maximum segment length = 100m

  9. 10BaseF • 10BaseF specification enable long distance connections with • the use of optical fiber. Three standards are: • 10BaseFP - A passive-star topology, up to 1 km link • 10BaseFL - An asynchronous point-to-point link • (a new FOIRL) , up to 2 km link • 10BaseFB - A synchonous point-to-point link , up to 2 km link • with 15 cascade repeaters

  10. Carrier Sense Multiple Access • Station listens to the medium before transmitting (listen before talking ;LBT) • Non persistent CSMA • if medium is idle, transmit • if medium busy, waits a random periods an then resense medium • 1-persistent CSMA • if medium is idle, transmit • if medium busy, continue to listen until the channel is sensed idle; then transmit immediately

  11. Media Access Control technique • Carrier Sense Multiple Access with Collision Detection (CSMA/CD) • listen before talking • Station senses to the medium before transmitting : CS • Topology supports multiple access (listenin) : MA • Manchester encoding ensures a transition every bit • 0 : high-to-low • 1 : low-to-high 0 1 0 0 1 1 0 0 0 1 1 0 mA 0 V (high) -90 mA -2.2 V (low) 100 ns

  12. Collision • More than two station send frame at the same time • Station transmits its data while checking the cahannel for collision • If average DC voltage level exeeds the CD threshold, collision is detected

  13. Collision detection • How to detect • Station send frame and sense the medium • Collision detected if • station sense the exeeded signal strength (coax) • there is signal on more than one inut (UTP) • How to do • Transmitting nodes send a jamming signal after a collision is detected. jam

  14. FCS Pad LLC PDU DA SA LEN PA SFD FCS Pad Data DA SA Type PA Ethernet frame format 7 1 6 6 2 46-1500 4 IEEE 802.3 calculation of the FCS 64-1518 bytes 8 6 6 2 46-1500 4 Ethernet PA : Preamble - 10101010s for synchronization SFD : Start of Frame delimiter -- 10101011 to start frame DA: Destination Address -- MAC address SA: Source Address -- MAC address LEN: Length -- Number of data bytes Type: identify the higher -level protocol LLC PDU+pad -- minimum 46 bytes, maximum 1500 FCS : Frame Check Sequence -- CRC-32

  15. Ethernet MAC address 1 1 46 I/G U/L 46-bit address Two addressformat: 48 bits 16 bits 1 15 I/G 15-bit address I/G =0 Individual address I/G =1 Group address U/L=0 Global administered address U/L=1 Local administered address Unicast : define a single destination Broadcast : FFFFFFFF each station on the network receive and accept frames multicast : a group address defines multiple recepient

  16. FCS Pad Data DA SA Type PA Interframe gap PA Interframe gap > 9.6 ms • Obligatory 9.6ms interval between the emitted frame and the new one • To enable other stations wishing to transmit to take over at this time

  17. Frame transmission Assemble frame carrier sense signal ON ? Y N Wait interframe gap time Start trnasmission Y send jam sequence Increment attempt. collision detected? N N Y transmission done? Compute backoff and wait backoff time attempt limited? Y N tranmit OK Discard frame

  18. Frame reception start receiving done receiving? N Y N Matched DA Y N Discard frame FCS and frame size OK? Y Pass frame to next layer

  19. Binary Backoff delay= 2 send frame N transmission done? collision detected? N Y Y random wait between 0 and delay-1 N tranmit OK >16 attempts 1st wait 0 or 1 slot time 2nd wait 0,1,2 or 3 slot time 3rd wait 0,1,2,..7 slot time kth wait 0.. 2k slot time Y Discard frame double delay • after 10th collisions the interval is frozen at 1023 slots • after 16th collisions , frame is discarded and report failure • back to the upper layer limited delay<1024 max delay is limited at 1023*51.2 ms=52.4 ms

  20. Minimum frame size A and B locates at the far end of the cable B B A (2) packet almost at B at t-d A (1) packet starts at time 0 (3) B send packet; collision occurs at t A B B (4) jam signal gets back to A at 2t A • A frame must take more than 2t to send to prevent the situation that the sender incorrect • conclude that the frame was successfully sent. • This slot times is = 51.2 ms corresponds to 512 bit = 64 bytes • The minimum frame length is 64 bytes (excluding preamble) • This answers why data field must have 46 bytes minimum

  21. Late Collision • Late collisions are collisions which occur after the first 64 bytes have been transmitted on to the network • Primary causes: excessive cable lengths and repeaters

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